As shown in FIG. 1, a conventional boost type LED driver 10 has an input terminal 14 to receive an input voltage VIN, and a pair of output terminals 16 and 18 for an LED string 12 to be connected therebetween. In this boost type LED driver 10, an asynchronous boost converter 20 steps up the input voltage VIN to a regulated voltage Vo to apply to the LED string 12, and a current source 22 is to be serially connected with the LED string 12 to set the driving current and thereby the brightness of the LED string 12. In the asynchronous boost converter 20, an input capacitor Ci is connected to the input terminal 14 to stabilize the input voltage VIN, an inductor L is connected between the input terminal 14 and a switching node 26, a diode D1 is connected between the switching node 26 and the output terminal 16, an output capacitor Co is connected to the output terminal 16 to stabilize the output voltage Vo, a MOS M1 is connected between the switching node 26 and a ground terminal GND, and a boost controller 24 switches the MOS M1 to regulate the output voltage Vo. During the on-time of the MOS M1, the inductor L is charged by the voltage source VIN, and during the off-time of the MOS M1, the inductor L releases energy to the output capacitor Co.
In FIG. 1, the LED string 12 includes three LEDs connected in series. If each of the LEDs requires a driving voltage of 3V and the current source 22 requires a driving voltage of 0.5V, the boost type LED driver 10 has to provide the output voltage Vo of 9.5V in order to drive the LED string 12. When the input voltage VIN is lower than 9.5V, for example 4.5V, the boost converter 20 will step up the input voltage VIN to the output voltage Vo of 9.5V. In this case, the boost type LED driver 10 operates in a boost mode and thus has a higher efficiency. However, since the conventional boost type LED driver 10 lacks of a buck mode for operation, when the input voltage VIN is higher than 9.5V, for example 21V, the boost converter 20 will stay idle and the MOS M1 will remain on, so the output voltage Vo will be approximately equal to the input voltage VIN, and the boost type LED driver 10 enters a low dropout (LDO) mode. In this case, the LED string 12 consumes only 9V and the left 12V of the power source is completely allotted to the current source 22. Therefore, most of power is consumed by the current source 22, leading the efficiency of the boost type LED driver 10 to be degraded, lower to about (9.5/21)×100%.
As shown in FIG. 2, a conventional inverting boost type LED driver 30 includes an input terminal 32 to receive an input voltage VIN, and a pair of output terminals 34 and 36 for an LED string 12 to be connected therebetween. In addition to the current source 22 to set the driving current of the LED string 12, this inverting boost type LED driver 30 includes an asynchronous inverting boost converter 38 to step up the input voltage VIN to a regulated voltage Vo having an opposite polarity to that of the input voltage VIN. In the asynchronous inverting boost converter 38, an input capacitor Ci is connected to the input terminal 32 to stabilize the input voltage VIN, a MOS M1 is connected between the input terminal 32 and a switching node 42, an inductor L is connected between the switching node 42 and a ground terminal GND, a diode D1 is connected between the switching node 42 and the output terminal 34 to prevent a reverse current from the switching node 42 to the output terminal 34, an output capacitor Co is connected to the output terminal 34 to stabilize the output voltage Vo, and an inverting boost controller 40 switches the MOS M1 to regulate the output voltage Vo.
In FIG. 2, the LED string 12 includes three LEDs connected in series. If each of the LEDs requires a driving voltage of 3V and the current source 22 requires a driving voltage of 0.5V, the inverting boost type converter 38 will be designed to provide the output voltage Vo of −9.5V. In the case that the input voltage VIN is 21V, the inverting boost type LED driver 30 will step down the input voltage VIN of 21V to an output voltage of 9.5V for the LED string 12. In other words, the inverting boost type LED driver 30 acts as operating in a buck mode.
FIG. 3 is a circuit diagram of the inverting boost type LED driver 30 operating in a boost mode. If the input voltage VIN is 21V and the LED string 12 includes ten LEDs, at least 30.5V is required for driving the LED string 12. In this case, the inverting boost type converter 38 provides the output voltage Vo of −30.5V, and the inverting boost type LED driver 30 acts as stepping up the input voltage VIN of 21V to an output voltage of 30.5V for the LED string 12. Therefore, the MOS M1 suffers a voltage drop of about 51.5V (=21−(−30.5)), and thus must be a high-voltage element able to sustain 51.5V. This requires higher cost for hardware. In addition, the higher the voltage drop of the MOS M1 is, the greater the current flowing through the MOS M1 and the inductor L is. As a result, the on resistance of the MOS M1 and the parasitic resistance of the inductor L cause higher power consumption, leading the efficiency of the inverting boost type LED driver 30 to be lower. Moreover, the output voltage Vo and the operation mode of the inverting boost type converter 38 are set by the design of the inverting boost converter 38, and can not be changed any longer once the circuit design of the inverting boost converter 38 is completed.